expression
someday; one day; one of these days
Expresses a hopeful or wistful expectation that something will happen at an unspecified future time. Often used in poetic or reflective contexts.
いつの日か、世界中を旅してみたい。
Someday, I want to travel around the world.
いつの日かまた会えると信じている。
I believe we will meet again one day.
いつか is the more common, everyday word for 'someday'. いつの日か is more emphatic and literary, often carrying a stronger emotional or poetic nuance.
A fixed phrase composed of いつ (when), の (possessive particle), 日 (day), and か (question particle), literally 'on what day?'. The combination has become a set expression meaning 'someday'.